1. Field of the Invention
The present invention relates to the field of signal processing, and, more particularly, relates to multi-channel demodulators for demodulating mixed signals, such as, for example, signals generated in a pulse oximetry system.
2. Description of the Related Art
In many multi-channel measurement and communication systems, crosstalk between channels and corruption of data within the channels are significant problems. Such problems can arise from variations in manufacturing tolerances, movement, propagation delays, phase shifts, temperature effects, degradation of components due to age or other factors, noise, etc.
A pulse oximetry system is one example of a system where the above-referenced problems are found. In a pulse oximetry system, blood oxygen saturation is determined by transmitting pulses of electromagnetic energy through a portion of a subject having blood flowing therein (e.g., through a finger, through an ear lobe, or other portion of the body where blood flows close to the skin). The pulses of electromagnetic energy comprise periodic pulses of red light having wavelengths of approximately 660 nanometers, for example, and periodic pulses of infrared light having wavelengths of approximately 905 nanometers.
After propagating through the portion of the subject, the red pulses and the infrared pulses are detected by a detector which is responsive to light at both wavelengths and which generates an electrical signal that has a relationship to the intensity of the electromagnetic energy incident on the detector. The detector output is a two-channel signal having a first signal component corresponding to the detected red pulses and a second signal component corresponding to the detected infrared pulses.
The two-channel signal is demodulated to recover separate signals corresponding to the first signal component and the second signal component. However, prior art demodulators are not sufficiently accurate enough to completely separate the two signal components in all cases. Thus, it is not uncommon for the first demodulator output signal (corresponding to the first signal component) to contain residual components of the second signal and vice versa. This crosstalk between the first and second signal components reduces the accuracy of the recovered first and second signals. In multi-channel systems with more than two channels, crosstalk can occur between all of the channels, again reducing accuracy.